pH in Pregnancy and Why Your Diet Will Not Change It

There is no shortage of trashy diet advice going around, whether in the blogosphere, in food stores, or on the pages of some best-selling books. A good portion of it is directed at women who are pregnant, or trying to get pregnant, and this includes a trend that has been around for several years, the so-called “alkaline diet”. Typically, the diet is pushed with phrases designed to make it sound scientific. In reality, however, there is nothing scientific about it, and adopting such a fad diet could prevent you from obtaining important nutrients.

The standard sales pitch goes something like this: The fetus is extremely sensitive to changes in pH, your body pH changes throughout pregnancy, pH imbalance underlies infertility and pregnancy complications (such as nausea/vomiting, heartburn, and swelling), and you can take control by reducing intake of “acid-forming foods” and eating more “alkaline-forming foods”. As part of the solution, you are directed to consume some product that is being sold. In one of dozens of anti-acid websites, for instance, the product for sale was some kind of alkaline mineral water.

The reality is that your body pH doesn’t change that much when you drink something that’s alkaline (high pH), and any change is very quickly reversed. If you consume antacid solution, such as bicarbonate, which is strongly alkaline (much more alkaline that any of the so-called “alkaline-forming foods), it will neutralize the acidity in your stomach –it will increase your stomach pH– for a time. This may alleviate heartburn, a symptom that results from stomach acid leaking upward into your esophagus. However, even a dose of antacid won’t increase your overall body pH significantly.

That’s a good thing, though, because our biochemistry is indeed very sensitive to pH.

But the fact that you and your fetus are sensitive to pH is not a rationale for adopting an “alkaline diet”. Our metabolism is so tuned to pH that we have evolved multiple systems to keep pH within an extremely tight range. This is something that happens with other phenomena, such as body temperature, blood pressure, and the balance between water and various electrolytes in cells and in body fluids, and it’s called homeostasis. Homeostatic mechanisms maintain pH of your blood at 7.4. If your pH drifts upward or downward by hundredths of a pH point, your body feels it and reacts to counter the change. If pH drops below 7.35, it’s called acidosis, whereas pH rising above 7.45 is alkalosis.

It’s normal to become mildly acidotic during vigorous exercise, and mildly alkalotic during certain other situations, such as ascending quickly to high altitude. In both situations, body systems then compensate and push the pH back toward 7.4. In the disease process, things can go wrong in pH homeostasis, especially when the disease involves the kidneys or lungs, because, as we shall see, these organs play major roles in pH balance. The digestive system can produce changes in body pH as well, through vomiting and diarrhea, but not on account of the pH of food that you consume.

So what then does take control of body pH? To begin, the blood itself resists changing pH, because of chemical buffers, consisting of the bicarbonate-carbonic acid system, phosphate ion buffers, and buffering from blood proteins, especially hemoglobin. Second, partly because reaction of the bicarbonate-carbonic acid system alters the blood concentration of carbon dioxide (CO2), the respiratory system also reacts very quickly. If pH begins to drop (if blood goes slightly acidic), your breathing rate increases, causing you to exhale CO2 faster than normal, thereby lowering CO2 concentration. On account of the bicarbonate-carbonic acid system, this decrease in CO2 then drives the pH back up to normal. Thus, if you exercise vigorously, producing lactic acid that begins to acidify the blood, the lungs will keep the pH in check. On the other hand, if something else, vomiting for example, makes your slightly alkaline, your breathing will slow, causing you to retain more than the usual amount of CO2, driving your pH back toward normal.

As for the kidneys, they control the concentration of bicarbonate and other ions in the blood. When operating normally, for instance as you acclimate to higher altitude, the kidneys will gradually accelerate the excretion of bicarbonate and decelerate the excretion of acids. This will compensate for the accelerated exhalation of CO2, but the change takes several hours to kick in. Because the kidneys operate by controlling the movement of molecules between the blood and urine, that there are things that you can take to make your urine more acidic, or more alkaline. Inside the body, however, pH depends on the kidneys working in concert with the lungs and other systems, so it is much harder to change the pH of your blood than of your urine. Temporary changes in blood pH can result from power events, like changes in your breathing rate, vomiting out your stomach acid, ascending a mountain quickly, exercising vigorously, or a medical problem with an organ that controls pH. But you don’t alter your pH based on the pH of your food.

So what about the vomiting and heartburn of pregnancy? Do these symptoms justify anything that the alkaline diet people are saying? Not really. Earlier we noted that heartburn is what you feel when stomach acid leaks into the esophagus. In pregnancy, it’s not the result of a pH imbalance, as the promoters of alkaline diet may tell you. It’s common, simply because the growing uterus is pushing on your stomach, causing acid to reflux, not because there is extra acid in your body. As for the vomiting, that’s part of what’s called NVP –nausea and vomiting of pregnancy. NVP is common during the first trimester, and is thought to result from a combination of mechanisms that have not been entirely elucidated. It’s related to pH in the sense that it can cause a temporary pH change, loss of acid, due to vomiting. This means, if anything, you might suffer slight alkalosis, in which case you should want the opposite of what alkaline diet is sold to promote.

Dr. David Warmflash is a science communicator and physician with a research background in astrobiology and space medicine. He has completed research fellowships at NASA Johnson Space Center, the University of Pennsylvania, and Brandeis University. Since 2002, he has been collaborating with The Planetary Society on experiments helping us to understand the effects of deep space radiation on life forms, and since 2011 has worked nearly full time in medical writing and science journalism. His focus area includes the emergence of new biotechnologies and their impact on biomedicine, public health, and society.